pdqdl......you are right their is a flaw and I indicated as so in my post. Like so many other threads on this site.....one's statement of opinion is taken as fact and thus the criticizing begins. Again, I am just saying, in my experience and my opinion.....I am happy with my stump grinders and I believe them to be a big part of my business. I know for a fact, after merging my business with another, not everyone has the same experience.
For me, I take the cost of the machine and double it and divide by the expected useful life of the machine (say 2000 hours) to come up with my cost per hour. Add operating costs, labor and overhead plus profit and I have what I charge per hour. This formula works for me.